gh-128863: Deprecate the private _PyUnicodeWriter API (#129245)

Deprecate private C API functions:

* _PyUnicodeWriter_Init()
* _PyUnicodeWriter_Finish()
* _PyUnicodeWriter_Dealloc()
* _PyUnicodeWriter_WriteChar()
* _PyUnicodeWriter_WriteStr()
* _PyUnicodeWriter_WriteSubstring()
* _PyUnicodeWriter_WriteASCIIString()
* _PyUnicodeWriter_WriteLatin1String()

These functions are not deprecated in the internal C API (if the
Py_BUILD_CORE macro is defined).